The speakers may be powered by a seperate amp, and that is what may have been on, even if the radio wasn't. Although, I would think the amp(s) have a turn-on lead?
Did ALL of the speakers make the noise or just one? Perhaps a pinched lead? One stereo trick I learned in the old days was to short the leads with a 9-volt battery...the associated speaker would make a "pop" sound, indicating that it was hooked to those leads. Helped to determine if the correct leads were connected to the correct speaker during installation. Perhaps a ground lead pinched against a "hot" wire somewhere?